Method for transferring a pattern

ABSTRACT

A method and a device for forming at least a first pattern on a substrate ( 6,28 ), which is coated with a layer ( 5 ) on at least a first planar surface, are described. According to the method, a first pressing means ( 27 ) is arranged with a first structure ( 32 ) that defines the pattern, and the substrate ( 28 ) is arranged so that the first pressing means ( 27 ) is separated from the substrate ( 28 ) and so that the first pressing means ( 27 ) and the substrate ( 28 ) are unable to move transversely of the common center line ( 50 ) but are able to move freely in a direction first pressing means ( 27 ) is contacted with the substrate ( 28 ) while maintaining the common center line, and a pressure is applied to the substrate ( 28 ) and the first pressing means ( 27 ) to form said at least one pattern in the layer ( 30 )

FIELD OF THE INVENTION

The present invention relates to a method and a device for transferringa pattern from a pressing means to an object.

BACKGROUND ART

Among other things in the semiconductor industry and in the manufactureof optical storage media, a pattern is to be transferred to an object.There are a number of different techniques for transferring a pattern toan object, such as optical lithography and electron beam lithography. Atechnique which has recently come into use and which, for instance, isused for manufacturing CDs (compact disks) is transferring a pattern bypressing.

PCT Application SE01/00527 discloses a device and a method fortransferring a pattern from a pressing means to a substrate, which isprovided with a layer, by means of pressing. In the application, atechnique to obtain an even pressure over the entire surface of thepressing means is described.

In the manufacture of, among other things, rotating storage media, it isimportant for the pattern which is being transferred to the object to becentered in relation to a central rotational axis.

However, SE01/00527 does not teach how the pattern can be centered inrelation to a rotational axis.

In the semiconductor technique, a pattern is often aligned to anotherpattern by means of alignment marks provided, respectively, on thesubstrate to which the pattern is to be transferred and on the mask fromwhich the pattern is to be transferred.

One example of a technical field with a great need for pattern alignmentis the manufacture of the new generation hard disks, in which thesurface has been formed with a structure to obtain a high storagecapacity. Traditional hard disks are provided with a magnetic layer onboth sides. When formatting the hard disk, magnetic areas are definedsimultaneously on both sides of the hard disk by means of a read/writefork on each side. In order to increase the storage capacity, the sizeof the magnetized areas has traditionally been reduced. These areas can,however, only be reduced to a certain limit before an area is affectedby adjacent areas to such a degree that the storage of data in one areachanges data in an adjacent area.

One way of solving this problem is to define magnetic areas in advancein each layer, the areas being separated by a non-magnetic material.

In this case, however, it is necessary for the magnetic areas on bothsides to be situated at the same radial distance from the rotationalaxis to enable the read fork to read and write on both sidessimultaneously. It is of less importance that the areas are not situatedin the same positions in the tangential direction.

Thus, there is a need for a method and a device for aligning a substratein relation to a pressing means with a structure.

There is also a need for a method and a device for aligning two pressingmeans in relation to each other and in relation to a substrate.

SUMMARY OF THE INVENTION

An object of the present invention is to solve at least one of theproblems described above.

It is also an object of the present invention to provide a method anddevice for aligning a pressing means in relation to a substrate.

Another object of the present invention is to provide a method and adevice for aligning two pressing means, each provided with a structure,in relation to each other and in relation to an object.

At least one of these objects is achieved by means of a method, a deviceand use according to the independent claims. Further features of theinvention will appear from the dependent claims.

It is a basic idea of the invention to use a mechanical alignment meansto align the pressing means and the substrate, the alignment not beingsensitive to variations in the geometric dimensions of the pressingmeans or the substrate.

A method according to the invention is used for forming at least a firstpattern on a substrate, which is coated with a layer on at least a firstplanar surface. The method is characterized in that it comprises thesteps of arranging in the following order a first pressing means, whichhas a first surface with a first structure that defines the firstpattern, and the substrate so that the center of the substrate and thecenter of the first structure are arranged on a common center line, andso that the first surface of the first pressing means is facing and isseparated from the first planar surface of the substrate and so that thefirst pressing means, the second pressing means and the substrate areunable to move transversely of the common center line but are able tomove freely in a direction along the center line. The method furthercomprises the steps of contacting the pressing means with the substratewhile maintaining the common center line, and applying a pressure to thesubstrate and the first pressing means to form said at least one patternin the layer.

By using a means with the above features, the pressing means and thesubstrate are automatically centered. It is relatively easy to providesuch alignment.

A method according to the invention can, of course, be adjusted so asalso to form a first pattern and a second pattern on each side of asubstrate. In this case, it is possible that it is desirable only tocenter the first pattern in relation to the second pattern. If so, thesubstrate does not have to be arranged so that it is unable to movetransversely of the common center line.

A method for forming a first pattern and a second pattern on asubstrate, which has at least a first and a second planar surface whichare coated with a first and a second layer, respectively, ischaracterized in that it comprises the step of arranging in thefollowing order a first pressing means, which has a first surface with afirst structure that defines the first pattern, the substrate and asecond pressing means, which has a second surface with a secondstructure that defines the second pattern, so that the centers of thefirst and the second structure are arranged on a common center line. Thepressing means and the substrate are arranged so that the first surfaceof the first pressing means is facing and is separated from the firstplanar surface of the substrate, so that the second surface of thesecond pressing means is facing and is separated from the second planarsurface of the substrate, and so that the first pressing means and thesecond pressing means are unable to move in relation to each otherperpendicular to the common center line but are able to move in adirection along the center line. After that, the pressing means arecontacted with the substrate while maintaining the common center line,and a pressure is applied to the first and the second pressing means toform the patterns in the layers.

If it is desirable to center the patterns also in relation to thesubstrate, also the substrate is arranged with its center on the commoncenter line.

It is possible to provide the alignment with the aid of a means whichacts on the peripheries of the pressing means and the substrate.However, it is only by means of a method, in which the pressing meansand the substrate are centered with the aid of through-holes in thepressing means and the substrate, that the full extent of the inventionis exploited and a less complicated method is obtained.

If the substrate and the pressing means are centered by means ofthrough-holes, a center through-hole is provided in the substrate, whichcenter hole has a center axis that extends through the center of gravityof the substrate and coincides with a normal direction of said planarsurfaces. The first pressing means has a first through-hole, which has acenter axis that coincides with a normal direction of said first surfaceof said first pressing means. If a second pressing means is provided,this is provided with a second through-hole, which has a center axisthat coincides with a normal direction of said second surface of saidsecond pressing means. The step of arranging the center of saidsubstrate and the centers of said first and, possibly, second structureson a common center line comprises arranging said substrate and saidfirst and, possibly, second pressing means on a mandrel, which isarranged to extend through said center hole, first hole and, possibly,second hole. A mandrel which is arranged to extend through said centerhole in the manner described above is much easier to manufacture than ameans which is arranged to engage the peripheries of the substrate andthe pressing means.

According to one embodiment, said center hole, the first hole and thesecond hole are uniform and preferably circular. The mandrel isrelatively easy to manufacture if it is intended for uniform holes andin particular if intended for circular holes.

According to one embodiment, the mandrel has a conical portion, inparticular a conical portion which is circular in cross-section. Thisshape is easy and thus cheap to manufacture.

According to one embodiment, the largest cross-sectional area of saidconical portion is larger than said first hole which is larger than saidcenter hole which is larger than said, possibly, second hole.Consequently, the active part of the mandrel consists of a conicalportion only. Naturally, it is possible as an alternative to use amandrel with several conical portions.

A method as stated above can be used in various ways. It can, forinstance, be used to manufacture two-sided rotating storage media or tomanufacture hard disks.

According to one embodiment of the-invention, a method is provided forforming a first pattern and a second pattern on a substrate, which hasat least a first and a second planar surface which are coated with afirst and a second layer, respectively. The method is characterized inthat it comprises the steps of arranging in the following order a firstpressing means, which has a first surface with a first structuredefining the first pattern and which has a first circular through-hole,the center line of which is perpendicular to the first surface and whichis centered in relation to the pattern, the substrate and asecond-pressing means, which has a second surface with a secondstructure defining the second pattern and which has a second circularthrough-hole, the center line of which is perpendicular to the firstsurface and which is centered in relation to the pattern, on a conicalmandrel of circular cross-section. The largest diameter of the mandrelis larger than the biggest hole. The pressing means and the substrateare arranged with the first surface of the first pressing means facingand being separated from the first planar surface of the substrate, andthe second surface of the second pressing means facing and beingseparated from the second planar surface of the substrate.

Subsequently, the pressing means are contacted with the substrate whilemaintaining the common center line, and a pressure is applied to thefirst and the second pressing means to form the patterns in the layers.

According to one embodiment of the invention, use is provided of aconical mandrel, which is circular in cross-section, to align at least afirst pressing means, with a first pattern, in relation to a substratewhen transferring the pattern to the substrate by pressing, the pressingmeans having a first circular through-hole which is centered in relationto the pattern and the substrate having a second circular through-hole.

According to yet another aspect of the present invention, a device isprovided for forming at least a first pattern from a first pressingmeans with a first structure to a plate-shaped substrate, which iscoated with a layer on at least a first planar surface. The devicecomprises a first holding means and a second holding means for receivingone of the substrate and the first pressing means each, the device beingarranged to apply a pressure between the first and the second holdingmeans. The device further comprises a centering means, which is movablein relation to the second holding means and which is arranged in such amanner that the center of the substrate and the center of the firststructure are arranged on a common center line, that the first planarsurface of the substrate is separated from the structure and that thefirst pressing means and the substrate are separated from the secondholding means and are unable to move perpendicular to the common centerline but are able to move freely in a direction along the center line,when the substrate and the pressing means are arranged on the centeringmeans, when the centering means is in a first working position, with thestructure facing the first planar surface. The device is also arrangedso that the first planar surface of the substrate is in contact with thestructure and arranged on the same center line, and one of the substrateand the pressing means is in contact with the second holding means whenthe centering means is in a second working position. The device isfurther arranged to contact the first holding means with thesubstrate/the pair of pressing means before a pressure is appliedbetween the first and the second pressing means.

By such a device it is relatively easy to transfer a pattern to thesubstrate. According to a further aspect of the invention, a device isprovided for forming at least a first pattern from a first pressingmeans with a first structure to a plate-shaped substrate, which iscoated with a layer on at least a first planar surface. The devicecomprises a first holding means and a second holding means for receivingone of the substrate and the first pressing means each, the device beingarranged to apply a pressure between the first and the second holdingmeans. The device is characterized in that it further comprises acentering means in the form of a cone, which is movable in relation tothe second holding means and which is arranged in such a manner that thefirst planar surface of the substrate is separated from the structureand that the first pressing means and the substrate are separated fromthe holding means, when the substrate and the pressing means arearranged on the centering means when the centering means is in a firstworking position, that the first planar surface of the substrate is incontact with the structure and arranged on the same center line, and oneof the substrate and the pressing means is in contact with the secondholding means when the centering means is in a second working position,the device being arranged to contact the first holding means with thesubstrate/the pair of pressing means before a pressure is appliedbetween the first and the second pressing means. By means of a devicewith a centering means in the form of such a cone, it is easy to centerthe substrate and the pressing means. A cone is easy to manufacture, forinstance, by turning. It goes without saying that it is conceivable toarrange a pressing means on each side of the substrate if it isdesirable to make a pattern on each side of the substrate.

The features which have been described in connection with the method canbe implemented also in the device after suitable adjustments.

Naturally, the features mentioned above can be combined in the sameembodiment.

Below embodiments of the invention will be described with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a method according to a preferred embodiment of theinvention.

FIG. 2 shows a pressing means according to a preferred embodiment of theinvention.

FIG. 3 shows a device according to an alternative embodiment of theinvention.

FIG. 4 shows a pressing means according to an embodiment of theinvention.

FIG. 5 shows a centering means according to an alternative embodiment ofthe invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates a method according to a preferred embodiment of theinvention. FIG. 1 shows a first holding means 1 and a second holdingmeans 2 between which a pressure can be applied to press together apressing means 4, which is provided with a structure 7, and a substrate6, which on a first planar surface 8 is provided with a layer 5. Betweenthe pressure means and the first holding means, a first sealing ring 3and a second sealing ring 13 are arranged to form a chamber 10 which canbe used to create a pressure by means of which the pressing means ispressed into the layer. The chamber 10 is formed when the first holdingmeans has contacted the sealing rings. The pressure is generated byinjecting high-pressure gas through the conduit 9. At the center of thesecond holding means 2, a centering means 14 protrudes which has aconical portion of circular cross-section at its top. In the pressingmeans, a first circular through-hole 12 is arranged which has the samediameter as the upper part of the centering means 14. In the substrate 6and its layer 5, a corresponding second through-hole 15 is made which islarger than the first circular through-hole 12. As a result, thesubstrate and the pressing means are held by the conical centering means14. The diameters of the first circular through-hole 12 and the secondcircular through-hole 15 as well as the conicity and the diameter of thecentering means 14 are adjusted to each other so that the substrate andthe pressing means 4 are kept in such positions that a gap is providedbetween the structure 7 on the pressing means 4 and the layer 5 on thesubstrate 6. The centering means 14 is advantageously made in one piecein a turning lathe, which enables the peripheral surfaces of thecentering means 14 to be satisfactorily centered.

When using the centering means 14, the substrate 6 and the pressingmeans 4 are arranged on the centering means 14, in this order, thestructure on the pressing means facing the layer 5 on the substrate 6.After that, the first holding means 1 is raised towards the secondholding means 2 so that the first holding means. 1 first contacts thesubstrate 6, that the layer 5 subsequently contacts the structure on thepressing means 4 and that the pressing means 4 finally contacts thesealing rings 3 and 13. The first holding means 1 moves in the axialdirection. Owing to this, the alignment between the pressing means 4 andthe substrate 6 is maintained.

After being brought together, a pressure is applied to the pressingmeans 4 by filling the chamber 10 with gas through the conduit 9.Thereby the pattern defined by the structure 7 is transferred to thelayer 5. The first through-hole 12 in the stamp is centered in relationto the second through-hole 15 in the substrate. As a result, thepattern, which is made in the layer 5, is centered in relation to thesecond through-hole 15 in the substrate.

FIG. 2 shows an alternative embodiment of the present invention. In FIG.2, the centering means is formed as a first partial means 20, a secondpartial means 21 and a third partial means 22, which are fixedlyarranged on a mounting pin 23. Each of the partial means has a conicalsurface 24, 25, 26 which is circular in cross-section. Each of theconical surfaces 24, 25, 26 has a symmetry axis which coincides withthat of the mounting pin 23 on which they are arranged. The conicalsurfaces 24, 25, 26 are machined when the partial means are arranged onthe mounting pin 23. It is thus possible to achieve good conformitybetween the symmetry axes of the conical surfaces and the symmetry axisof the mounting pin 23.

In FIG. 2, a first pressing means 27 is arranged on the first partialmeans 20, a substrate 28 is arranged on the second partial means 21 anda second pressing means 29 is arranged on the third partial means 22.The first pressing means 27 is provided with a first structure 32, onthe side facing the substrate 28. The substrate 28 is provided with afirst layer 30, which is facing the first pressing means 27, and asecond layer 31 which is facing the second pressing means 29. The secondpressing means is provided with a second structure, on the side facingthe substrate 28.

The first pressing means 27 is provided with a first through-hole 34.The substrate is provided with a second through-hole 35. The secondpressing means 29 is provided with a third through-hole 36. Thethrough-holes are arranged in relation to the partial means in such amanner that the substrate 28 and the pressing means 27, 29 each getstuck to one of the partial means 20, 21 and 22. After having centeredthe pressing means 27, 29 and the substrate 28 by placing them on thecentering means, they are brought together in a manner corresponding tothat described in connection with FIG. 1 and subsequently the firstpressing means is pressed together with the second pressing means sothat the pattern is transferred to the first layer 30 and the secondlayer 31 on the substrate 28. The structure 32 on the first pressingmeans 20 is centered in relation to the first through-hole 34. Thestructure on the second pressing means 22 is similarly centered inrelation to the third through-hole 36. As a result, when pressing thefirst pressing means and the second pressing means together with thesubstrate in between them, the patterns which are made in the firstlayer 30 and the second layer 31, respectively, will be centered inrelation to each other.

FIG. 3 shows a device according to the invention in the form of a presswhich can be used to transfer a pattern to a layer on a substrate. Thedevice has a first holding means 37 which is fixedly arranged inrelation to the frame structure 38 of the device, a second holding means39 which is movable in the vertical direction in relation to the firstholding means and which is arranged to apply a static pressure in thevertical direction. Centrally in the second holding means 39, acentering means 40 is arranged, which is, for instance, of the kinddescribed in connection with FIG. 1. When using the press in FIG. 3, thefollowing elements are arranged as described with reference to FIG. 2: afirst pressing means 41 with a first structure arranged on the sidefacing upwards, a substrate 42 with a layer on each side and then asecond pressing means 43 with a second structure arranged on the sidefacing downwards.

After having placed the substrate 42 and the pressing means 41, 43 onthe centering means, the second holding means 39 is raised so that thepressing means 41, 43 and the substrate 42 contact each other and areclamped between the first holding means 37 and the second holding means39. Subsequently, a pressure is applied between the first holding means37 and the second holding means 39 so that the structures in thepressing means are transferred to patterns in the layers.

FIG. 4. shows a pressing means 44 of circular shape through which athrough-hole 45 is made. On the surface of the pressing means, astructure is arranged in the form of concentric circular raised portions46. The hole 45 is centered in relation to the raised portions.

In FIG. 5, two alternative forms of centering means are shown. Thedifferent forms shown in FIGS. 5 a and 5 b can be used to provide equaldistance between the substrate and each one of the pressing means whenthe difference in diameter varies between the through-holes in thesubstrate and the pressing means, respectively. The embodimentsdescribed above are merely examples to enable the one skilled in the artto work the invention and they do not limit the invention which isdefined by the appended claims.

1. A method for forming at least a first pattern on a substrate, whichsubstrate has at least a first planar surface which is coated with afirst layer comprising the steps of: arranging the substrate, which hasa center hole, and a first pressing means having a first surfaceprovided with a first structure defining the first pattern and furtherprovided with a first through-hole, the center line of which isperpendicular to the first surface and centered in relation to thepattern, on a conical mandrel extending through and in engagement withsaid center hole and in engagement with said first through-hole, withthe first surface of the first pressing means facing and being separatedfrom the first planar surface of the substrate, and the second surfaceof the second pressing means facing and being separated, contacting thepressing means with the substrate while maintaining a common centerline, and applying a pressure to the first pressing means to form thefirst pattern in the first layer.
 2. A method for forming at least afirst pattern and a second pattern on a substrate, which substrate hasat least a first planar surface and a second planar surface which arecoated with a first layer and a second layer, respectively, comprisingthe steps of: arranging a first pressing means, having a first surfaceprovided with a first structure defining the first pattern provided witha first through-hole, the center line of which is perpendicular to thefirst surface and centered in relation to the pattern; the substrate,which is provided with a center through-hole; and a second pressingmeans, having a second surface provided with a second structure definingthe second pattern and which is provided with a second through-hole, thecenter line of which is perpendicular to the second surface and centeredin relation to the second pattern, on a conical mandrel extending fromand in engagement with said second through-hole, through said centerthrough-hole, into and in engagement with said first through hole, withthe first surface of the first pressing means facing and being separatedfrom the first planar surface of the substrate, and the second surfaceof the second pressing means facing and being separated from the secondplanar surface of the substrate, contacting the pressing means with thesubstrate while maintaining a common center line, and applying apressure to the first pressing means and the second pressing means toform the patterns in the layers.
 3. A method as claimed in claim 2,wherein said mandrel is arranged in engagement with said centerthrough-hole, wherein also the substrate is arranged with its center onthe common center line.
 4. A method as claimed in any one of claims 1-3,for manufacturing a two-sided rotating storage medium.
 5. A method asclaimed in claim 4, for manufacturing hard disks.
 6. A method forcentering a substrate provided with a layer on a first planar surface,in relation to a pressing means having a first surface provided with afirst structure defining a first pattern, in a process for transferringsaid first pattern to said layer by means of pressing, comprisingproviding a first hole is provided in the center of said firststructure, providing a center hole is provided in said substrate,providing a mandrel with a conical portion about a center axis, andbringing the mandrel through and into engagement with said center holeand said first hole, such that said first planar surface and said firstsurface are arranged parallel to and spaced apart from each other with acommon center line along said center axis.
 7. (cancelled).
 8. A devicefor forming at least a first pattern from a first pressing means with afirst structure on a plate-shaped substrate which substrate is coatedwith a layer on at least a first planar surface, comprising a firstholding means and a second holding means for receiving one of thesubstrate and the first pressing means each, the device being arrangedto apply a pressure between the first holding means and the secondholding means, and a centering means, which is movable in relation tothe second holding means and which is arranged in such a manner that thecenter of the substrate and the center of the first structure arearranged on a common center line, that the first planar surface of thesubstrate is separated from the structure and that the first pressingmeans and the substrate are separated from the second holding means andare unable to move perpendicular to the common center line but are ableto move freely in a direction along the center line, when the substrateand the first pressing means are arranged on the centering means, whenthe centering means is in a first working position, with the structurefacing the first planar surface, and that the first planar surface ofthe substrate is in contact with the structure and arranged on a commoncenter line, and one of the substrate and the pressing means is incontact with the second holding means when the centering means is in asecond working position, the device being arranged to contact the firstholding means with the substrate/pressing means pair before a pressureis applied between the first holding means and the second holding means.9. A device for forming at least a first pattern from a first pressingmeans with a first structure on a plate-shaped substrate, whichsubstrate is coated with a layer on at least a first planar surface,comprising a first holding means and a second holding means forreceiving one of the substrate and the first pressing means each, thedevice being arranged to apply a pressure between the first holdingmeans and the second holding means, and a centering means in the form ofa mandrel with a conical portion, which is movable in relation to thesecond holding means and which is arranged in such a manner that thefirst planar surface of the substrate is separated from the structureand that the first pressing means and the substrate are separated fromthe holding means, when the substrate and the pressing means arearranged on the centering means when the centering means is in a firstworking position, and that the first planar surface of the substrate isin contact with the structure and arranged on a common center line, andone of the substrate and the pressing means is in contact with thesecond holding means when the centering means is in a second workingposition, the device being arranged to contact the first holding meanswith the substrate/pressing means pair before a pressure is appliedbetween the first holding means and the second holding means.
 10. Adevice for forming at least a first pattern and a second pattern on asubstrate, which substrate has at least a first planar surface and asecond planar surface which are coated with a first layer and a secondlayer, respectively, comprising a first pressing means, having a firstsurface provided with a first structure defining the first pattern, anda second pressing means, having a second surface provided with a secondstructure defining the second pattern and facing said first surface,wherein a first hole, the center line of which is perpendicular to thefirst surface and centered in relation to the pattern, is arranged inthe first pressing means; a second hole, the center line of which isperpendicular to the second surface and centered in relation to thesecond pattern, is arranged in the second pressing means; the substrateis provided with a center through-hole; and centering means in the formof a mandrel having a conical portion about a center axis, which mandrelis displaceable in relation to the second pressing means, is introducedfrom and in engagement with said second hole, through said centerthrough-hole, into and in engagement with said first hole, such thatsaid structures are arranged parallel to and spaced apart from eachother, with a common center line along said center axis.
 11. A devicefor centering a substrate provided with a layer on a first planarsurface, in relation to a pressing means having a first surface providedwith a first structure defining a first pattern, for transferring saidfirst pattern to said layer by means of pressing, wherein a first holeis provided in the center of said first structure, and a center hole isprovided in said substrate, and wherein a mandrel provided with aconical portion about a center axis is arranged through and inengagement with said center hole and said first hole, such that saidfirst planar surface and said first surface are arranged parallel to andspaced apart from each other with a common center line along said centeraxis.
 12. A device for centering a first pressing means having a firstsurface provided with a first structure defining a first pattern, inrelation to a second pressing means having a second surface providedwith a second structure defining a second pattern, for use in a processwhere a substrate provided with a first layer on a first planar surfaceand a second layer on a second planar surface, is placed between saidpressing means for transferring said first and second patterns to saidfirst and second layers by means of pressing, wherein a first hole isprovided in the center of said first structure, a center hole isprovided in said substrate, and a second hole is provided in the centerof said second structure, and wherein a mandrel provided with a conicalportion about a center axis is arranged through and in engagement withsaid first hole, through said center hole, and in engagement with saidsecond hole, such that said first and second surfaces are arrangedparallel to and spaced apart from each other with a common center linealong said center axis.
 13. A device as claimed in any one of claims8-12, wherein said center hole and said first hole are uniform.
 14. Adevice as claimed in claim 10 or 12, wherein said center hole, firsthole and said second hole are uniform.
 15. A device as claimed in claim13, wherein said holes are circular.
 16. A device as claimed in claim15, wherein said conical portion is circular in cross-section.
 17. Adevice as claimed in claim 14, wherein said holes are circular.
 18. Adevice as claimed in claim 17, wherein said conical portion is circularin cross-section.